Archive for April, 2011


The search for ever more intriguing aromatic/antiaromatic species continues on – Haley has recently prepared the TIPS-protected indeno[1,2-b]fluorene 1. 1 The crystal structure was determined and analogue with the tri-iso-propylsilyl groups replaced with hydrogens (2) has been computed at B3LYP/6-31G(d,p). This optimized structure is shown in Figure 1. The core system has 20 π-electrons – suggesting perhaps an antiaromatic system.



Figure 1. B3LYP/6-31G(d,p) optimized geometry of 2.

The x-ray structure and computed structure are in close agreement in terms of distances. The terminal phenyl rings exhibit very little alternation. The C1-C2 and C2-C3 distances are long (1.444 and 1.457 Å, respectively) while the C1-C3A and C­2-C4 distances are short (1.379 and 1.396 Å, respectively.) This suggests a para-xylylene type structure for the central six-member ring. The NICS values of the terminal 6-member ring, the 5-member ring and the central 6-member ring are computed to be -7.12 (a reasonable phenyl value), 1.84, and 0.02. So the middle three rings possess no aromatic or antiaromatic character. Haley describes this structure as “a fully conjugated 20-π-electron hydrocarbon with fused s-trans 1,3-diene linkages across the top and bottom portions of the carbon skeleton”.


(1) Chase, D. T.; Rose, B. D.; McClintock, S. P.; Zakharov, L. N.; Haley, M. M., "Indeno[1,2-b]fluorenes: Fully Conjugated Antiaromatic Analogues of Acenes," Angew. Chem. Int. Ed., 2011, 50, 1127-1130, DOI: 10.1002/anie.201006312


1: InChI=1/C64H92Si4/c1-41(2)65(42(3)4,43(5)6)37-33-57-53-29-25-27-31-55(53)61-60(36-40-68(50(19)20,51(21)22)52(23)24)64-58(34-38-66(44(7)8,45(9)10)46(11)12)54-30-26-28-32-56(54)62(64)59(63(57)61)35-39-67(47(13)14,48(15)16)49(17)18/h25-32,41-52H,1-24H3

2: InChI=1/C28H12/c1-5-17-21-13-9-11-15-23(21)27-20(8-4)26-18(6-2)22-14-10-12-16-24(22)28(26)19(7-3)25(17)27/h1-4,9-16H

Aromaticity Steven Bachrach 20 Apr 2011 No Comments

Describing hydrogen bonds with DFT

Proper handling of hydrogen bonding using DFT remains a concern. Sherrill has reported a careful benchmark study using the potential energy curves for the six dimer combinations involving formic acid, formamide, and formamidine.1 Comparisons are made to the the CBS extrapolated limit CCSD(T) curve.

As anticipated, B3LYP and related functionals do a poor job. Interestingly, PBE and PBE0 provide very nice curves. The meta-GGA functionals like M05-2x and M06-2x and functionals with dispersion corrections (like ωB97X-D) provide very good potential energy curves. It is clear that intermediate and long-range correlation and dispersion must be accounted for when handling hydrogen bonded systems. Proper selection of the functional is critical.


(1) Thanthiriwatte, K. S.; Hohenstein, E. G.; Burns, L. A.; Sherrill, C. D., “Assessment of the Performance of DFT and DFT-D Methods for Describing Distance Dependence of Hydrogen-Bonded Interactions,” J. Chem. Theory Comput., 2011, 7, 88-96, DOI: 10.1021/ct100469b.

Uncategorized Steven Bachrach 05 Apr 2011 No Comments